A reception terminal or transmission terminal of an integrated circuit is coupled to a termination resistor having the same resistance value as characteristic impedance of a transmission channel. The termination resistor matches impedance of the reception terminal or transmission terminal with the characteristic impedance of the transmission channel, thereby suppressing reflection of signals transmitted through the transmission channel. Conventionally, the termination resistor has been installed outside a semiconductor chip. Recently, however, an on-die termination (ODT) circuit having a termination resistor installed inside a semiconductor chip has been used. The ODT circuit includes a switching circuit configured to control a current flowing therein through an on/off operation. Therefore, the ODT circuit requires smaller power consumption than a termination resistor installed outside a chip. The ODT circuit has a resistance value which changes depending on a PVT (Process, Voltage, and Temperature) variation. Therefore, before the ODT circuit is used, an impedance calibration circuit is applied to calibrate the resistance value of the ODT circuit.
The impedance calibration circuit includes a comparator configured to compare a resistance value of an external resistor coupled to a ZQ pad with a reference voltage, and the impedance calibration circuit generates a pull-up code and a pull-down code for calibrating the resistance value of the ODT circuit. Here, the external resistor coupled to the ZQ pad has a constant resistance value (generally, 240Ω) regardless of a PVT variation.
Meanwhile, the comparator may be used in an integrated circuit as well as the above-described calibration circuit, and performs an operation of comparing input voltages to output a logic level. However, an offset occurring in the comparator during a wafer manufacturing process may cause an error in the operation of the comparator to compare the input voltages. Furthermore, when the comparator in which an offset occurred is used in an impedance calibration circuit, impedance mismatching occurs, which makes it difficult to transmit data at high speed. Furthermore, output data may be distorted.